Sir2 represses endogenous polymerase II transcription units in the ribosomal DNA nontranscribed spacer
Sir2 represses endogenous polymerase II transcription units in the ribosomal DNA nontranscribed spacer
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DOI：
10.1091/mbc.e06-03-0205
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发表时间：
2006-09-01
影响因子：
3.3
通讯作者：
Bryk, Mary
中科院分区：
文献类型：
--
作者：
Li, Chonghua;Mueller, John E.;Bryk, Mary
酿酒酵母中rDNA、HM位点和端粒的沉默需要组蛋白修饰酶来产生对重组和RNA聚合酶II转录机制难治的染色质结构域。为了探讨沉默因子Sir 2如何调节rDNA上染色质的组成和功能，通过染色质免疫沉淀法测定了组蛋白和RNA聚合酶II与rDNA的结合。我们发现，Sir 2不仅调节K4-甲基化组蛋白H3在rDNA的水平，但也总组蛋白H3和RNA聚合酶II的水平。此外，我们的研究结果表明，Sir 2限制甲基化组蛋白在rDNA的能力，需要其脱乙酰酶活性。在sir 2 Delta细胞中，rDNA非转录间隔区的高水平K4-三甲基化H3与RNA聚合酶II在非转录间隔区中的转录单位表达以及先前未检测到的染色质结构改变相关。总之，这些数据表明一个模型，其中Sir 2的脱乙酰酶活性防止rDNA的常染色质化，并沉默天然存在的基因间转录单位，其表达与内聚复合物的破坏和rDNA的重复扩增有关。
Silencing at the rDNA, HM loci, and telomeres in Saccharomyces cerevisiae requires histone-modifying enzymes to create chromatin domains that are refractory to recombination and RNA polymerase II transcription machineries. To explore how the silencing factor Sir2 regulates the composition and function of chromatin at the rDNA, the association of histones and RNA polyrnerase II with the rDNA was measured by chromatin immunoprecipitation. We found that Sir2 regulates not only the levels of K4-methylated histone H3 at the rDNA but also the levels of total histone H3 and RNA polymerase II. Furthermore, our results demonstrate that the ability of Sir2 to limit methylated histones at the rDNA requires its deacetylase activity. In sir2 Delta cells, high levels of K4-trimethylated H3 at the rDNA nontranscribed spacer are associated with the expression of transcription units in the nontranscribed spacer by RNA polymerase II and with previously undetected alterations in chromatin structure. Together, these data suggest a model where the deacetylase activity of Sir2 prevents euchromatinization of the rDNA and silences naturally occurring intergenic transcription units whose expression has been associated with disruption of cohesion complexes and repeat amplification at the rDNA.